1. Field of the Invention
The present invention relates to a thin film transistor, and particularly relates to a thin film transistor suitable for a case where its semiconductor layer is an oxide semiconductor layer.
2. Description of the Related Art
As described in Unexamined Japanese Patent Application KOKAI Publication No. H5-67786, an inversely staggered structure is generally adopted for a thin film transistor used as a switching element of an active matrix liquid crystal display device. A thin film transistor of this structure is constructed such that a gate electrode is formed on an insulating substrate, a gate insulating film is formed on the upper surface of the gate electrode and insulating substrate, a semiconductor thin film made of intrinsic amorphous silicon is formed on the upper surface of the gate insulating film that is on the gate electrode, a channel protection film is formed on the center portion of the upper surface of the semiconductor thin film, ohmic contact layers made of n-type amorphous silicon are formed on both sides of the upper surface of the channel protection film and on the semiconductor thin film at both sides of the channel protection film, and source and drain electrodes are formed on the upper surfaces of the respective ohmic contact layers.
Recently, there is an idea of using zinc oxide (ZnO) instead of amorphous silicon, because a higher mobility than that of amorphous silicon can be obtained from zinc oxide. In a case where a zinc oxide film is formed by CVD (Chemical Vapor Deposition), the film property is unstable in the initial state. Therefore, if the inversely staggered structure is adopted, the zinc oxide film in this unfavorable initial state is placed opposite to the gate electrode, i.e., the zinc oxide film in the unfavorable state forms the channel region, making it harder to achieve excellent properties as a thin film transistor. A cure for this that is now being considered is to make a thin film transistor of a top gate type, in which the upper surface of the zinc oxide film serves as the channel region. A conceivable manufacturing method of a thin film transistor of the top gate type using zinc oxide is to, for example, form a semiconductor thin film forming layer made of intrinsic zinc oxide on a gate insulating film, form a patterned channel protection film made of silicon nitride on the upper surface of the semiconductor thin film forming layer, form an ohmic contact layer forming layer made of n-type zinc oxide on the upper surface of the semiconductor thin film forming layer including the channel protection film, sequentially pattern the ohmic contact layer forming layer and the semiconductor thin film forming layer to form an ohmic contact layer and a semiconductor thin film in the device area, and form source and drain electrodes by patterning on the upper surface of each patterned ohmic contact layer.